High molecular weight aromatic polycarbonate resins are generally obtained from an interfacial or phase boundry process or by means of interesterification or by a combination of both processes. Such processes are disclosed in U.S. Pat. No. 3,028,365.
In these processes, diphenol is employed as one of the major reactants and is either supplied to the polycarbonate manufacturer or produced by him at his plant site. In the latter instance, the crude diphenol, prior to use, is distilled and then recrystallized in a suitable solvent. The solvent is then filtered and crystalline diphenol is collected. The material which does not crystallize and which remains with the filtrate is concentrated in a solvent stripper. The concentrated material is dark colored and, in addition to containing diphenol, also contains a large portion of diphenol by-products. Although diphenol by-products can be employed to produce high molecular weight aromatic polycarbonate resins, the polycarbonate resins obtained are also dark colored and of inferior quality.
Some of the by-products contained in the diphenol can be used efficiently and effectively in the production of polycarbonate resins. For example, chroman which is typically found in such by-products, can be used as a molecular weight regulator or chain stopper in producing high molecular weight aromatic polycarbonates as disclosed in U.S. Pat. No. 3,697,481.
The isolation of these by-products can, therefore, be advantageous, but they must be obtained in sufficient quantity and purity for use and the process employed should be economical if it is to be practical. U.S. Pat. No. 3,825,562 discloses an aqueous process for purifying such by-products employing sodium hydroxide. However, this process is not practical as it is time consuming and the relative quantity of purified by-products obtained, e.g., chroman, is not sufficient from a practical, manufacturing view point.